1. Cross-Reference to Related Applications
This application is a continuation-in-part of U.S. application Ser. No. 07/011,736 filed Feb. 6, 1987, abandoned. This application is related to application Ser. No. 07/088,429 filed Aug. 24, 1987, now U.S. Pat. No. 4,862,184, which describes a physical structure embodying the principles of the invention described in the present application.
2. Field of the Invention
This invention relates to helical antenna structures and more particularly to resonant monopole antennas of helical configuration and to the electronic tuning of such antennas.
3. Description of the Related Art:
Various kinds of electrically short (less than onequarter wavelength long) antennas have been used both with and without top loading and with and without electrical tuning, U.S. Pat. No. 4,656,483 to Jaquet describes an antenna switchable between UHF and VHF bands. The antenna comprises a capacitive element spaced from a ground surface, an inductance inserted between the capacitive element and a crossing point of the ground surface that is connected to a transmitter or receiver. The antenna is adjusted for VHF band use by short-circuiting a portion of the inductance and for UHF use by short-circuiting all of the turns of the inductance. A second inductor is connected between the first inductance and the ground surface for VHF band use and is disconnected for UHF band use. Conductive side members are inserted between the capacitive member and the ground surface on opposite sides of the inductors. The antenna is restricted in practical application by the need for additional components (blocking self-inductances), bias voltage requirements and the size necessary to accommodate the capacitive radiating elements.
U.S. Pat. No. 4,564,843 to Cooper describes another capacitive radiating element connected through a series of tuning inductors to the transmitting source. Each inductor is connected to a pair of P.I.N. diodes capable of short circuiting the inductor in order to tune the antenna to different frequencies. The antenna is not a helical antenna and the inductors do not themselves serve as radiating elements. In this structure, the feeding arrangement is unbalanced with respect to the tuning coils so that very high r-f currents flow on the bias leads resulting in undesirable losses. The blocking self inductances ("chokes") L10---L33 are an absolute essential in an antenna with the described mode of operation.
U.S. Pat. No. 4,554,554 to Olesen describes an axial mode helical antenna with four separate and relatively long radiating elements surrounding a central support that are connected or disconnected by P.I.N. diodes. Antennas of this type radiate from the end with a directional pattern.